High power fuse with ceramic casing



Jan. 24, w RATH HIGH POWER FUSE WITH CERAMIC CASING Filed Oct. 13, 1965 F/G. I

lNVENTOP.

ER/CH WERNER RATH United States Pate'ntO 3,300,608 HIGH POWER FUS WITH CERAMIC CASING Erich Werner Rath, Hersbruckerstrasse 22, Lauf, Pegnitz, Germany I Filed Oct. 13, 1965, Ser. No. 495,592 Claims priority, application Germany, June 15, 1965,

St 23,980 9 Claims. (Cl. 200-131) ed primarily to porcelain, or the known steatite compounds. After tests it has been found that, due to heating, heat cracks or fissures may appear. The strength of steatite, however, was considered to be so high that fuses which did show these fissures were still accepted for use.

- The fissures themselves usuallywere not of sufiicient breadth to permit an external break-through of a flash, ora flame when the fuse itself had to interrupt very high current, such as a short-circuit current.

Various standards acknowledge the presence of such heat fissures. For example, Germ-an published standards VDE 0660 of December 1952, and particularly sections 36A and 64F, and standard'DIN 43.620 January 10, 1960, permit fuses showing such fissures to be used. Nevertheless. fuses with such fissures were not as effective as desired, and if the fissures were excessive, returns and rejects caused waste in production and criticism in use. The fusible link within the fuse is often surrounded by quartz sand to fill the space there between and the casing. The linkis then secured to metal plates, in turn fastenedto the casing. When a fuse has passed maximum rated current for a long period of time, then, upon occurrence of a su-bstantial'overload or a short circuit, the fuse may heat to such an extent that a fuse body which already does have heat fissures ruptures and the now hot quartz sand flows out requiring bothersome cleaning. Removal of the remaining parts of the fuse body from the fuse holder, particularly if the parts are still hot, is very difiicult.

According to the present invention, a fuse is provided which has a casing which will not rupture even under the most severe operating or test condition, by forming the ceramic casing essentially entirely of sintered cordierite. The cordierite should be dense, and, for certain applications, is preferably coated with a glazing, for

example by a self-glaze. Surprisingly, it has been found that cordierite, although it has only about half the tensile strength of steatite or other customarily used ceramic materials, has unusual and particular advantages as housings for fuses. Contrary to the prevailing view, that such housings must be formed of a material having the highest possible tensile strength, for example steatite, it has been found that the use of the cordierite as a material for the housing for casing of the fuse entirely avoids heat fissures in the body of the casing, and thus avoids the predisposition to rupture under operating, or test conditions. Fuses having casings formed of steatite and sub jected to a test current for 30 minutes were found to fissure, even before thermal equilibrium has been reached; in contrast thereto, and with identical test conditions, fuses having housings formed of cordierite remained free of fissures long after thermal equilibrium has been 3,3btifi08 Patented Jan. 24, 1967 reached. The substantially greater ability of cordierite housings to endure rapid temperature rises due to internal heating of the fusible link, without fissuring or rupturing, is equally advantageous in comparison with porcelain based fuses. It is thus no longer necessary to accept fuses with heat fissures even though they may meet some official test standards.

The cordierite substance of the fuse casing may consist of the known three or four component systems MgO-Al O -SiO and, if desired, further containing an alkali oxide additive as fourth component, to provide for self-glazing. Fuse casings formed from cordierite which is compression sintered, so that it will not absorb moisture in its structure which might reduce the insulating properties, are particularly useful. Further, it has been found that the thermal resistivity is particularly high if the insulating material is sintered, that is fired, until glazing just about commences.

When the four component system is used, that is utilizing for example K 0 or Na O; MgO; A1 0 SiO for the casing of the fuse in accordance with the present invention, then it is preferably fireduntil self-glazing be gins. The insulating body then has a smooth, glazed, glossy external coating. It does not require a separate glaze coating and a second firing is avoided.

The structure, organization and operation of the invention will now be described more specifically in the following detailed description with reference to the accompanying drawing, in which:

FIG. 1 is an illustrative embodiment of a fuse having knife blade contacts;

FIG. 2 is a longitudinal section through the ceramic casing of the fuse of FIG. 1;

FIG. 3 is a top view of the ceramic casing;

FIG. 4 illustrates the fusible link without the casing in front view; and 5 FIG. 5 is an end view of the link of FIG. 4.

Fusible link 5, illustrated in FIGURES 4 and 5 (FIG- UR'E 5 showing the link after rupture) is inserted in the housing as well known in the art, surrounded, if desired by quartz sand. Metal covers 6 are secured to link 5 and assembled to casing 4, preferably by means of an intervening asbestos gasket 3. Contact blades 1 are preferably welded to metal covers 6. The entire assembly, that is metal covers 6, fusible link 5 and casing 4 are secured together by screws 2. The usual length of the fuse casing 4 is between 2 and 3 inches. I

Comparative tests were made with fuses of 65 mm. length, having a heating element of 154 watts built into their center. Fuses having casings made out of steatite showed fissures after heating for 26 to 28 minutes, that is, substantially before the period when thermal equilibrium is reached. Similar fuses having cases made of. cordierite without selfaglazing withstood heating for .37 minutes before rupturing. Fuses made with casings of cordierite having selfglazin-g withstood the same heating for over minutes, that is, after thermal equilibrium had been reached, and then still did not fail. The tests indicated that cordierite casings for fuses are better than cases made of 'steatite; and that cordierite casings having a self-glaze outer coating are substantially superior to the unglazed cordierite casings. Use of the self-glaze has the additional advantage that the smooth glazed surface represents a high surface resistance even in dirty or contaminated surroundings, without even requiring a further ad ditional step in the process of manufacture, mainly an additional glazing step.

The advantages of the composition above described obtain for low tension as well as for high tension fuses. In each case, the composition of the ceramic material used with or without self-glaze.

for the casing has a coefficient expansion and is formed from the preferred self glazinlg cordierite material, approximately within the following limits:

Example 1 Percent SiO 40 to 70 A1 20 to so .MgO 3 to 30 K 0 or a O 2 to 20 In the preferred form the material has the following composition:

Example 2 Percent SiO 53 A1 0 37 M gO 8 KNaO n, 2

Example 3 Percent SiO .52 A1 0 40 MgO 8 The firing temperature is the sameas for the self-glaze above, that is 1300 to 1340 C.

It is immaterial which raw materials are used toproyide the supply of silicic acid, alumina, magnesia and alkali. As a rule, ka-olian, clay, steatite and talc are suitable, although the components may be added inthe form of oxide as for example salts which form, oxides during firing (for example chlorides and sulphates, etc.), hy-

vdroxides,'and oxyhydrates. In a preferred form, the alkali is entirely or largely introduced as feldspar; alkali containing .fr-i'ts, or the like may also be used. A similar favorable effect can be obtained, as is known, with four component'imaterials if the magnesium oxide is wholly or partly replaced by zinc oxide or beryllium oxide, or a mixture of the two. Themagnesium oxide may also be replaced in part by calcium oxide, strontium oxide or barium oxide. The firing and dense sintering or c0mpression sintering, up to theforrnation of the self glaze on the surface on the body, if desired, is done in accordance with known ceramic methods, in which the firing temperature, firing time, and oven atmosphere is controlled in accordance with the desired materials, so that, when glazing is desired, the glazing will form before the work pieces' begin to soften, or the expansion introduces diificulties. Simple preliminary experiments in accordance with known ceramic techniques readily will show the correct conditions for various sizes and compositions of material.

I claim:

1. An electrical fuse comprising a tubular casing; a pair of cover means, each, closing a respective end of said casing; a pair of terminal elements, each element extending from a respective end of said casing and beyond a respectivecover meansyfusible strip means inside said casing conductively connecting said terminal elements; said casing enveloping and being spaced from said fusible strip means; said casing being-composed essentially' of cordierite to withstand overload heating without developing cracks.

2. Fuse as claimed in claim 1, said casing having a self-glazed surface.

3. Fuse as claimed in claim 1 wherein said cordierite material 'is based on the composition including MgO A1 0 SiO 4. Fuse as claimed in claim 1 wherein saidcordierite material further includes an alkali oxide additive to provide for surface glazing.

5. Fuse as claimed in claim 4 wherein said casing 'or cordierite material has been fired at a temperature of from 1300 to 1340 C.

6. Fuse as claimed in claim 1 wherein said cordierite material is densely sintered.

7. Fuse as claimed in claim 1 wherein said cordierite material has the following composition:

' Percent SiO- 40 to A1 0 20 to 50 MgO 3 to 30 K 0 or Na O 2 to 20 8. Fuse as claimed in claim 1 wherein said cordierite material has the following composition:

. Percent SiO 53 A1 0 37 MgO 8 KNaO c- 2 9. Fuse as claimed in claim 1 wherein said cordierite material has the following composition:

Smothers et al. 174-209 X OTHER REFERENCES Levin ,et'al.: Phase Diagrams for C erami sts, 1964 ed.,

vColumbus, Ohio, The American Ceramic Society, Inc,

pp.246 and 272.

BERNARD A. GILHEANY, Primary Examiner.

H. GILSON, Assistant Examiner. 

1. AN ELECTRICAL FUSE COMPRISING A TUBULAR CASING; A PAIR OF COVER MEANS, EACH CLOSING A RESPECTIVE END OF SAID CASING; A PAIR OF TERMINAL ELEMENTS, EACH ELEMENT EXTENDING FROM A RESPECTIVE END OF SAID CASING AND BEYOND A RESPECTIVE COVER MEANS; FUSIBLE STRIP MEANS INSIDE SAID CASING CONDUCTIVELY CONNECTING SAID TERMINAL ELEMENTS; SAID CASING ENVELOPING AND BEING SPACED FROM SAID FUSIBLE STRIP MEANS; SAID CASING BEING COMPOSED ESSENTIALLY OF CORDIERITE TO WITHSTAND OVERLOAD HEATING WITHOUT DEVELOPING CRACKS. 